Method and apparatus for removing distillate from physical solvents

ABSTRACT

Water is mixed with a stream of physical solvent used for acid gas removal which contains distillate. The mixture separates into an aqueous phase containing the physical solvent and water and an organic phase containing the distillate. The two phases are separated mechanically and the water and solvent are separated by fractional distillation.

11d Sites Finns 1191 egvveir METHUD AND APPARATUS FOR RlEMQi/HNGlDllTlllLlLATll 11 1101141 PHYSHQAL ILVENTS [75] inventor:

[73] Assignee: Fish Engineering & (Ionstrnction,

1111s., Houston, Tex.

22 Filed: .1611. 2a, 1972 Arnold M. illegwer, Houston, Tex.

[52] 111.3. 1. 55/45, 55/48, 55/68 [51] int. 1C1 Wild 119/00 [58] Fieldof Search 55/45, 48, 68, 73,

[56] References (liked UNITED STATES PATENTS Stotler 55/48 DIST/LLATE TOSTORAGE SOLVENT FROM TREA TING PLANT SOLVENT TO TREA TING PLANT June 19,1973 3,253,390 5/1966 (3611116118 Q. 55 45 x 3,254,473 6 1966 Fryar eta]. 55 45 x 3,492,788 2 1970 Hochgesandl et al. 55/48 3,542,196 11 1970Madlung, Jr 55 45 x Primary ExaminerSamih N. Zaharna AssistantExaminer-R. W. Burks Attorney-Paul E. Harris, Lee R. Larkin, Marcus L.Thompson et a1.

[5 7] ABSTRACT Water is mixed with a stream of physical solvent used foracid gas removal which contains distillate. The mixture separates intoan aqueous phase containing the physical solvent and water and anorganic phase containing the distillate. The two phases are separatedmechanically and the water and solvent are separated by fractionaldistillation.

4 (31211115, 11 Drawing Figure ATMO SPHERE METHOD AND APPARATUS FORREMOVING DISTILLATE FROM PHYSICAL SOLVENTS BACKGROUND OF THEINVENTION 1. Field of the Invention Solvent regeneration methods andapparatus for use in connection with acid gas removal from natural orsynthetic gas streams using physical solvents.

2. Description of Prior Art Processes utilizing physical solvents toremove acid gas constituents from a natural gas stream are wellknown inthe gas processing art. The physical solvents absorb the acid gasconstituents, such as CO H 8 and other sulphur compounds, from the gas.The solvents containing the absorbed acid gas are then removed from thenatural gas constituents herefrom, and the solvent stream is returned tocontact the natural gas stream to re-absorb acid gas again.

Problems have been experienced, however, in utilizing a physicalabsorption process of this type with natural gases containing distillatebecause the distillate is absorbed by the physical solvent. The termdistillate, as used herein, means heavier hydrocarbons of the molecularweight of pentane and higher.

The presence of a large portion of distillate in the physical solventaffects the ability of the solvent to absorb acid gas. The higher theconcentration of retained distillate in the flashed solvent, the lowerthe ability of the diluted solvent to absorb acid gas. This loweredabsorptive capacity requires higher solvent circulation rate, andincreased absorption unit capital expense to absorb the same amount ofacid gas. Additionally, the presence of distillate in the solventincreases the quan tity of methane absorbed in the solvent. Methane, aswell as ethane, butane and propane, are absorbed in the entraineddistillate and are carried over to be flashed off with the acid gas inthe flash unit. Any methane flashed in the acid gas recovery unit islost and cannot be utilized for sales gas without further recoveryoperations.

Separation of the distillate from the physical solvent by fractionaldistillation is not possible, since a portion of the distillate has thesame boiling range as the solvent in which it is entrained.

SUMMARY OF THE INVENTION The invention is a method and apparatus forremoving distillate from physical acid gas solvents. The method includesthe steps of mixing water with a water soluble acid gas physical solventcontaining distillate, holding the mixture to form an aqueous and anorganic phase, separating the aqueous phase obtained in the mixing stepwhich contains the solvent from the organic phase containing thedistillate and separating the solvent from the water.

The addition of water to the solvent containing distil' late results ina layering, or two phase liquid-liquid separation, the bottom layer ofwhich is the physical .solvent and water. The upper layer of the liquidis the organic layer containing the distillate which was formerlyhomogenously mixed with the solvent. The solvent and water are thenseparated by fractional distillation, and the solvent returned to theacid gas absorption installation.

BRIEF DESCRlPTION OF THE DRAWINGS The inventors presently preferredembodiment of the invention is shown in schematic form in the drawing.

DESCRIPTION OF THE PREFERRED EMBODIMENT Theinventors preferredembodiment of the invention is shown in the drawing. In operation oftheprocess, solvent from a natural gas treating plant, which has firstbeen contacted with a natural gas stream to absorb acid gas constituentstherefrom, then flashed to remove the low boiling constituentstherefrom, enters the separation process from the left hand side of thedrawing through a conduit 11. The physical solvent contains somedistillate dissolved therein on entering conduit 11 and most of the acidgas physically absorbed from a natural gas stream (not shown) haspreviously been flashed from the solvent. The solvent next passesthrough heat exchanger 12 in which the solvent is heated by exitingregenerated solvent from line 30 to a temperature of about F. Thesolvent then flows through line 13, wherein water is injected from line20 and the mixture proceeds to mixer 14 wherein the water and solventcontaining some distillate are intimately mixed. The mixed distillate,water, and solvent then flow through line 15 to a distillate separator16 and the mixture is held there until a two phase separation occurs,distillate in the upper layer thereof and solvent and water in the lowerlayer.

The upper layer, or organic layer, containing distillate is then drawnoff, or decanted, through a line 17 and into a lower portion of thedistillate contactor 18. Water is added through a line 19' in an upperportion of distillate contactor 18 and trickles down through the packedcolumn (not shown) in distillate contactor 18. Intermediate thedistillate entry point from line 17 and the water entry point from line19, a water level (not shown) is maintained within distillate contactor18 so that the distillate entering through line 17 will percolate upwardthrough the layer of standing water, continue its upward path throughthe packed column portion (not shown) of distillate contactor 18 andfinally exit through line 21 to a storage area (not shown).

All but trace amounts of solvent are removed from the distillate layerwithin the distillate separator 16, and distillate contactor 18 removesthe remaining traces of solvent from the distillate which is then takento storage. Although the physical acid gas solvent is soluble indistillate, it is more soluble in water, and when washed or mixed withwater the solvent will preferentially be dissolved in the aqueous phase.Standing water from the lower portion of the distillate contactor 18 iswithdrawn through line 22, passed through pump 23, and

returned through line 20 to be mixed with incoming solvent from heatexchanger 12.

The lower layer of liquid containing water and solvent in distillateseparator 16 is withdrawn from a lower portion of distillate separator16 and proceeds through line 24 through heat exchanger 25.wherein it isheated by the bottoms from the solvent still 26 prior to enteringsolvent still 26 through line 27.

Within solvent still 26, the mixture of water and phys ical solvent isfractionally distilled. Solvent still 26 may be of any standard design,and for a system of the size shown in Table I below, 10-15 trays aresufficient for adequate separation of the solvent and water. The bottomsof still 26 are the physical solvent, and the vapors drawn off the upperportion of still 26 are water vapor.

Heat for solvent still 26 is provided by a re-boiler 28 with appropriateconduits 31, 32 for extracting a portion of the liquid from the stilland returning the heated liquid back to solvent still 26. The bottoms ofsolvent still 26, containing the fractionated physical solvent, areextracted from a lower portion of solvent still 26 by line 33, passedthrough pump 34, line 35 and heat exchanged in heat exchanger 25 withthe separated solvent-water mixture from distillate separator 16. Fromheat exchanger 25, the regenerated solvent is returned by way of line 30heat exchanger 12 and line 36 to the acid gas removal plant (not shown).

Water vapor leaving solvent still 26 through line 37 is condensed atheat exchanger 38 and flowed into a re flux accumulator 29. Makeup waterfrom line 40 may be added to reflux accumulator 29 to replace lossesoccuring during the various plant operations. Any water vapor notcondensed at heat exchanger 38 or reflux accumulator 39 is then removedby appropriate pumping means 41, such as a jet adductor or compressor,and vented to the atmosphere Water accumulated in reflux accumulator 39exits a lower portion thereof through line 42, passes through pump 43and line 44 to enter an upper portion of solvent still 26 through line45 as reflux. The major portion of water flowed through 44, however,continues through line46 through heat exchanger 47 and line 19todistillate contactor 18.

A material balance for a 145 gallon per minute plant total solvent flowis given below in Table l at the cited points in the system in gallonsper minute, with the temperatures and pressures at those points given indegrees F and pounds per square inch absolute, respectively.

ing acid gas constituents include the dimethyl ethers of diethyleneglycol, triethylene glycol, tretraethylene glycol, pentaethylene glycol,hexaethylene glycol, heptaethylene glycol, and polyethylene glycol, aswell as solvents such as N-methyl-2 pyrrolidone, propylene carbonate andmethyl cyano acetate. This enumeration of specific solvents is not meantto limit the scope of the invention, and any water soluble physicalsolvent which possess good acid gas absorption characteristics wouldperform satisfactorily in the process of the invention.

In most plants utilizing the invention, only a small portion of the flowof solvent containing some distillate contaminant will need to betreated by the present invention to adequately control build-up ofdistillate therein. For example, a gas treating plant with an inlet gasvolume of 275 MMSCF/day of natural gas containing 42 percent carbondioxide was operated to reduce the carbon dioxide content in the salesgas to 3.5 percent. The present invention was utilized with a processhaving only 3 percent of the total solvent circulation from the acid gasextraction plan passing therethrough. Without the benefit of the presentirivention, distillate concentration in the solvent reached aconcentration of percent. With the desired embodiment of the inventionin operation, the concentration of distillate in the solvent streamnever rose above 5 percent.

Thus it can be seen that an improved apparatusand method for removingdistillate from physical acid gas solvents has been shown. Furthermodifications and alternative embodiments of the invention will beapparent to those skilled in the art in view of this description.Accordingly, this description is to be construed as illus- TABLE IStream number ii 20 17 21 24 44 4s 33 Flow (GPM) of:

Distillate 7 7 7 Water t .30 30 33 3 Trace Solvent I38 Trace Trace l3 8l3 8 Total Flow (GPM) 145 30 7 7 168 33 3 138 Temperature (F) 45 130 160130 160 165 165 295 Pressure (PSlA) 6O 45 37 4O 7 7 The parameters givenin Table l are merely for illustrative purposes. Parameters may varydepending upon the operating temperature of the acid gas extraction unitfrom which the solvent flows to the present process, upon the particulartype of solvent being process, and the pressures which are found to bemost efficient for the particular embodiment of the invention. Forexample, solvent still 26 may be operated at a higher pressure withresultant increase in the bottoms temperature for line 33. However, thelower the pressure at which the solvent still 26 is operated, the lessdegradation that will occur to the solvent during the fractionaldistillation thereof. It has also been found that distillate separatorl6 and distillate contactor 18 may be operated at temperatures fromambient to 200F. with no appreciable loss of efficiency for the overallprocess. Reflux accumulator 39 may also be operated at temperaturesbetween ambient and 200 the optimum temperature and pressure beingdictated by the operating temperature of the solvent still, theparticular solvent used, and considerations of thermodynamic efficiency.

Examples of the physical solvents capable of absorbtrative only and isfor the purpose of teaching those skilled in the art the manner ofcarrying out the invention. It is to be understood that the form of theinvention herewith shown and described is to be taken as the presentlypreferred embodiment. Various changes may be made in the shape, size andarrangement of equipment. For example, equivalent elements or materialsmay be substituted for those illustrated and described herein, steps maybe reversed, and certain features of the invention may be utilizedindependently of the use of other features, all as would be apparent toone skilled in the art after having the benefit of this description ofthe invention.

1 claim:

1. ln an acid gas removal process, the combination of steps comprising:

contacting a gas stream containing acid gas and some organic distillatewith a hydrophyllic physical solvent to absorb the acid gasconstituents;

flashing the acid gas constituents from said solvent;

adding water to said solvent containing distillate;

holding said mixture of water and solvent in a quiesdecanting saidorganic phase from said aqueous phase; and, fractionally distilling saidaqueous phase to separate said solvent from said water. 4. The processas claimed in claim 1 wherein said adding step includes: adding water toabout 0.1 to about 10 percent by volume of the total amount of saidphysical solvent passing from said contacting step.

2. The process as claimed in claim 1, including the additional step of:returning said solvent to contact said gas stream.
 3. The process asclaimed in claim 1 wherein said separating steps together include:decanting said organic phase from said aqueous phase; and, fractionallydistilling said aqueous phase to separate said solvent from said water.4. The process as claimed in claim 1 wherein said adding step includes:adding water to about 0.1 to about 10 percent by volume of the totalamount of said physical solvent passing from said contacting step.